Structural Versatility and Energy Difference of Salt–Water Complex NaCl(H₂O) Encoded in Cryogenic Photoelectron Spectroscopy

A weakly bound complex usually has multiple structural isomers with small energy differences. The sophisticated ab initio calculations are the main workhorse for providing theoretical results of different isomers. In contrast, the experimental determination of the energy difference is very rare. We report the energy-difference measurement of a model complex: salt–water complex NaCl­(H₂O). We measured the energy difference among the structural isomers of the negatively charged NaCl­(H₂O) complex and the neutral counterpart using cryogenic photoelectron spectroscopy. The temperature-dependent photoelectron spectra (15–300 K) revealed that the negatively charged NaCl­(H₂O) and the neutral counterpart both have three isomers. The two higher-lying isomers are 186(22) and 481(48) cm^–1, respectively, above the most stable isomer for the negatively charged and 123(10) and 1821(24) cm^–1 for the neutral. These results provide a benchmark for the development of theoretic methods of weakly bound complexes. The experimental technique demonstrated here can be employed to investigate other weakly bound complexes with multiple isomers.